The present invention relates to an optical fiber cable for use in communications and measurement and, in particular, an optical fiber cable having a coated optical fiber cable and a method for manufacturing the same, as well as an optical fiber coil using the optical fiber cable.
The optical fiber cable using the optical fiber is excellent over a copper-incorporated cable core in terms of its non-conduction, small diameter, light weight and flexibility and advantageous over it in terms of its low loss, broad bandwidth, etc. For this reason, the development of the cable as a new communications transmission path has been progressed and it is fastly accepted over a wider range of application.
Further, the optical fibers are not affected by a thunder-stroke and electromagnetic noise and the development of optical fiber-incorporated sensors for measuring the voltage, current and magnetic field has now been under way. An optical fiber type magnetic field sensor using the Faraday effect may be listed by way of example. This sensor is placed in the magnetic field with the optical fiber cable in a coiled state and the rotation of polarization of a light beam passing through the optical fiber is measured to obtain the strength of a magnetic field from that rotation angle. Thus, the phenomenon of the rotation of polarization of a light beam in a magnetic field is called as the Faraday effect.
With reference to FIG. 12, explanation will be given below about an optical fiber cable using a single mode fiber capable of transmitting one mode in an available wavelength, as one example of the optical fiber cable for use in communications and measurement. The optical fiber 1 comprises a core 2 for allowing a light beam to be actually transmitted therethrough and a cladding 3 provided around the core 2 and somewhat smaller in refractive index than the core 2. As the material for the core 2 and cladding 3 use is widely made of quartz. Any slight injury, being produced in the quartz surface, grows under a tension force, temperature expansion/contraction, moisture penetration, etc., into an eventual destruction.
In the manufacturing process of the ordinary optical fiber 1, a protective coating layer 4 is provided around the cladding 3 to provide an optical fiber cable. As a result, it is possible to prevent the generation of such an injury in the optical fiber resulting from the contacting of moisture and dirt in the air and hence to achieve enhanced reliability. As a material for the protective coating layer 4 use is made of an ultraviolet-curing type resin or thermosetting type resin such as silicone rubber.
The single mode optical fiber cable as set out above has its inner optical fiber 1 (cladding 3 and core 4) compressed by the thermal expansion of the protective coating layer 4 and involves undesired dispersion and birefringence. In order to handle this problem, the twisting of the optical fiber has been widely practiced. The reason is that the specific twisting of the optical fiber allows the dispersion to be reduced to zero and ensures the long-distance communications.
Further, owing to the twisting of the optical fiber and hence the reduction of the birefringence, the optical fiber type magnetic field sensor utilizing the Faraday effect can accurately measure the magnetic field.
In the case where such twisting is imparted to the optical fiber 1, the retaining of the twisting is achieved by fitting a connector over each corresponding end of the optical fiber or applying an adhesive to each corresponding end.
However, the optical fiber cable as set out above has encountered the following problems. That is, with the optical fiber 1 fitted as set out above so as to retain the twisting of the optical fiber, there arises a risk of a dispersion/birefringence occurring at that fitting site. In the case where the optical fiber is used in a bent state, a radially small curve is liable to be produced locally, thus providing a cause for birefringence. Further, the twisted optical fiber 1, unless being so mounted as to be retained at all times under a constant tension force, there is a possibility that the twisted optical fiber will be entangled.
In the case where, as the material for the protective coating layer 4, use is made of a thermosetting type or room temperature-curing silicone, the following problem is presented. That is, since the thermosetting type silicon rubber is cured at high temperature to cover the optical fiber therewith, there have sometimes the cases that, in use under the ordinary temperature, stress (compression) acts upon the optical fiber 1 under the compression of the silicone rubber and hence a dispersion and birefringence arise. Further, even in the room temperature-curing type silicon rubber, the optical fiber 1 is also compressed at a time of curing, thus providing a cause for dispersion and birefringence.